1. Field of the Invention
This invention relates to a photosensitive member having an amorphous silicon photoconductive layer, and more particularly to a hydrogen-containing amorphous silicon photoconductive layer which is useful for electrophotographic copying machines, semiconductor laser beam printers and other applications.
2. Description of the Prior Art
Recent years have seen a surge of research regarding the application of amorphous silicon (hereinafter referred to briefly as a-Si), obtainable by glow discharge decomposition or sputtering, to solar cells, and some such products have been available on the market for some time.
In parallel with the above development, the application of a-Si to an electrophotographic sensitive member has been gathering attention. This is because a-Si is by far superior to the conventional selenium and CdS photosensitive members with respect to freedom from environmental pollution, resistance to heat and abrasion, and other properties. However, the manufacturing technology for a-Si solar cells cannot be directly applied to the production of a-Si photoconductive layers for electrophotography. This is because, whereas the solar cell need only have a dark resistance of about 10.sup.3 to 10.sup.4 ohms.multidot.cm, it is generally necessary that a photoconductive layer for electrophotography have a dark resistance in excess of 10.sup.13 ohms.multidot.cm.
Under the circumstances, Published Unexamined Japanese Patent Application Sho No. 54-145539 discloses an electrophotographic photoconductive layer formed with a-Si produced by glow discharge decomposition or sputtering and containing 10 to 40 atomic percent of hydrogen, 0.1 to 30 atomic percent of oxygen and, if necessary, 10.sup.-6 to 10.sup.-3 atomic percent of impurity of Group IIIA (inclusive of boron) of Periodic Table of the Elements or of Group VA (inclusive of phosphorus) of the same Table. However, when the present inventors actually incorporated the above proportions of hydrogen and impurities, and especially 0.1 atomic percent or more of oxygen, in a-Si and examined the overall electrophotographic characteristics of the resulting layer, it was found that, while the dark resistance of the a-Si was increased to a level suitable for electrophotography, its photosensitivity deteriorated significantly as the oxygen content was increased, and even at an oxygen level of 0.1 atomic percent, the photosensitivity was markedly lower than the conventional photosensitive member in the visible region of the spectrum.
As will be described in detail hereinafter, a-Si contains a fair amount of hydrogen because it is produced from SiH.sub.4, Si.sub.2 H.sub.6, Si.sub.3 H.sub.8 or the like as the starting material, B.sub.2 H.sub.6 or the like is employed when a Group IIIA impurity is incorporated, and in glow discharge, hydrogen is sometimes used as the carrier gas. Hydrogen from these sources combine with the Si in the a-Si layer in various modes. The infrared absorption spectrum of an a-Si for solar cell use shows absorptions in the wavenumber region of about 1900 to 2100 cm.sup.-1, but its absorption peak lies at 2000 cm.sup.-1. This wavenumber of 2000 cm.sup.-1 corresponds to the absorption peak of Si-H bonds, while the wavenumber corresponding to Si--H.sub.2 bonds is about 2090 cm.sup.-1. As will become apparent from the subsequent explanation, the infrared absorption coefficient ratio, .alpha.(2090 cm.sup.-1) to .alpha.(2000 cm.sup.-1), is an important factor in a-Si photoconductive members for electrophotography. Thus, if the ratio of the absorption coefficient at the wavenumber of 2090 cm.sup.-1 for Si-H.sub.2 bonds to the absorption coefficient at the wavenumber of 2000 cm.sup.-1 for Si-H bonds is outside of a given range, either the dark resistance of a-Si is considerably reduced or its photosensitivity is sacrificed. Referring to the a-Si solar cell mentioned hereinbefore, since its dark resistance may be as low as 10.sup.3 -10.sup.4 ohms.multidot.cm and its photosensitivity is also not so critical, it does not matter if the absorption at SiH (2000 cm.sup.-1) is substantially larger than that at SiH.sub.2 (2090 cm.sup.-1).
However, the a-Si for electrophotography must have high dark resistance and photosensitivity, as well as several other important characteristics, all of which cannot be attained by the conventional production methods. Furthermore, the structure of a-Si varies a great deal with different production methods and conditions, and there has been demanded an a-Si photoconductive layer which would also be excellent in production reproducibility and stability.